The present invention relates to optical modules and more particularly to the structure of an optical module as a terminal device of a wavelength division multiplexing optical transmission type or of a single-fiber bidirectional transmission type which transmits light of a plurality of wavelengths over a single optical fiber, and also a method of manufacturing the optical module.
In a recent information communication field, it is on its way to a rapid spread of upgrading communication traffic for high speed optical transfer of a large amount of data. In particular, a wider band of communication in access line has been accelerated with an explosive spread of the internet, and a remarkable increase in the market of FTTH (Fiber To The Home) service is seen. Among FTTH optical transmission systems, a PON (Passive Optical Network) system, in which a plurality of subscribers share a single fiber, is currently being increased in demand. In this system, data transmitted from a vendor's local station via a single fiber is branched by a splitter into 16 to 32 fibers, via which the data is distributed to respective subscribers, thus remarkably reducing the fiber laying cost. An ONU (Optical Network Unit) is installed on the side of each subscriber as a terminal device, and using wavelength-division multiplex (WDM), a down-link signal (having a wavelength of 1.5 μm) from the vendor's local station to the subscriber side and an up-link signal (having a wavelength of 1.3 μm) from the subscriber side to the vendor's local station are multiplexed to transmit the up- and down-link signals over the same fiber. A duplexer optical module is built in the ONU. The duplexer optical module is basically made up of a laser diode for transmission of the up-link signal, a photo detector for reception of the down-link signal, and a WDM filter for separating the up- and down-link signals.
Prior art module systems are roughly classified into two groups as follows. FIG. 12 shows a basic arrangement of a BIDI (BI-DIrectional) module of a system belonging to a first group including optical components of an LD 135, a PD 132, and a WDM filter 137 spatially arranged in a package 138. In this system, since the optical components can be manufactured independently of one another, a good manufacturing yield can be easily secured. This system can advantageously have a stable optical coupling efficiency, because lenses 131 and 134 integrated in CAN packages 133 and 136 are optically aligned with a fiber 130 while optical elements 135 and 132 mounted on the CAN packages 133 and 136 are operated, that is, because the lenses can be optically connected with the fiber 130 in a so-called active alignment. However, since this system requires an increased number of necessary components and an increased number of processing steps, the system becomes disadvantageous when it is desired to make the system compact and to reduce a cost.
FIG. 13 shows a basic arrangement of a PLC (Planar Lightwave Circuit) module of a system belonging to a second group, including respective optical components mounted on a waveguide substrate. In this system, when compared with the aforementioned BIDI module, such optical components as optical elements 141, 143 (for example, 141 being an LD and 143 being a PD) and a filter 142 can be integrated on the same platform substrate 140. Thus the module of the system can be made compact with a less number of necessary components and a less number of processing steps. In this PLC system, however, optical alignment is carried out based on only the alignment accuracies of the respective optical components while the system monitors alignment marks provided on the platform substrate 140 for the respective optical elements, that is, the system is of a so-called passive alignment scheme. Thus a mounting margin for causing the alignment accuracies of the optical components to be simultaneously satisfied becomes small and therefore it is difficult to secure a good manufacturing yield. Further, since light to be transmitted is propagated through an optical waveguide 145 mounted on the platform substrate 140, the characteristics of this system are more influenced by the propagation loss in the optical waveguide or by a change in the environmental temperature than the spacial light transmitting system. In addition, since the system has an optical connection with a low loss, an optical fiber 144 is required to be mounted close to the optical waveguide 145. For this reason, it is difficult to vacuum the optical module package and to secure a high reliability.
An example of a mounting structure of a duplexer optical module different from the above two prior systems is disclosed in JP-A-2006-154535. This optical module is shown in FIGS. 14A and 14B respectively as its perspective view and top view. The optical module of the illustrated example includes a substrate 1100 having a first tilted surface structure 1100a etched in a first direction and a second tilted surface structure 1100b etched in a second direction at an angle of 90 degrees with the first direction; a wavelength demultiplexer 1500 of a cubic shape arranged on the substrate for transmitting light or reflecting light by an angle of 90 degrees according to the wavelength; a laser-diode-side lens element 1400a arranged in the first tilted surface structure; a photo-detector-side lens element 1400b arranged in the second tilted surface structure; a laser diode 1200 arranged in the vicinity of an end of the first tilted surface structure for emitting light, passing the light through the laser-diode-side lens element and the wavelength demultiplexer, and externally outputting it; and a photo detector 1300 arranged in the vicinity of the end of the second tilted surface structure for receiving external light through the wavelength demultiplexer and the photo-detector-side lens element.
With such an arrangement, since optical elements can be collectively mounted on the same substrate as in the aforementioned PLC system, the optical module can be made compact. Because of its excellent productivity, the module can be inexpensively manufactured. In addition, since light transmitted between the fiber and the optical elements is propagated in a space on the substrate through the lens elements 1400a, 1400b and the wavelength demultiplexer, this module can avoid such a problem as mentioned above that characteristics are influenced by a propagation loss of the optical waveguide medium or by a change in the environmental temperature.